In radio communication between two devices, the radio channel on which the two devices communicate may not be constant, but instead change dynamically. Further, two different radio channels may individually differ substantially with respect to current conditions of the respective radio channel so that configurations used for a first radio channel may be improper to use for a second radio channel.
In order to cope with the varying conditions, different measures are taken in order to improve the radio performance between the two devices.
Channel and interference estimation play a key role in the overall performance of a network. Through the above estimates, a communication device, such as e.g. a User Equipment, UE, may perform proper signal demodulation and report channel quality level (e.g. Channel Quality Indicator, CQI, to another communication device, such as e.g. a Radio Base Station, RBS. An RBS may be used for doing proper link adaptation (LA). Both channel and interference estimates are derived from a set of Resources Elements (REs) transmitted from the RBS. The density, in frequency and time, of these REs is typically fixed or predetermined and similar for all UEs independent of the current radio channel conditions of each UE. This may lead to excessive overhead by over dimensioning the density of these REs. It may also lead to lower performance due to poor channel estimation due to lack of these REs. For example, if a Resource Signal (RS) pattern is designed to support a high speed scenario, e.g. 300 km/h, then the RS time density is over-dimensioned for a stationary scenario. Furthermore, RS frequency density must cope with high frequency selectivity scenarios, which makes it over-dimensioned for the frequency flat case. In order to cope with these different requirements, some sort of average or good-enough solution may be used. In other words, in order not to waste resources, the density of the REs or RSs may be defined so that the more extreme scenarios, as e.g. a high speed scenario of 300 km/h, may not be supported.
Channel and interference estimation for demodulation and CQI estimates are performed over a set of predetermined dedicated REs, or RSs i.e. REs carrying RSs. In 3rd Generation Partnership Project, 3GPP Long Term Evolution, LTE, technology, different releases use different sets of these REs. In LTE Release 8, each RBS transmits a set of REs called Cell-specific RSs (or CRS). CRSs are transmitted regularly in every subframe and occupy 8 REs per resource block, RB per antenna port. The CRSs are cell specific and they can occur in three different frequency shifts. The same pattern is used for all UEs connected to a specific RBS. Among other operations, CRSs are used in demodulation and CQI estimation.
In the demodulation process, channel and noise estimation takes place in three steps. In the first step, CRSs are used to analyse the channel by estimating Doppler frequency, SNR and channel time dispersion. These quantities are then used for defining the filter banks that are used in time and frequency channel filtering. In the second step, estimation of channel on the CRS positions takes place. Based on these estimates, interpolation along frequency and time direction occurs for estimating the channel on all the rest REs. In the final step, noise estimation on the CRS positions takes place. Thus the density of CRSs plays an important role in the performance of channel and interference estimation.
As stated above, in order not to waste too many resources, an operator may choose not to support the more extreme cases by determining a density that will cover at least most of the scenarios likely to occur. One drawback suffered is that resources are wasted for many scenarios where the conditions are better than the worst case which is supported. The fact that reference signal patterns should be designed to support the most challenging scenario in which the network should operate, and support various network operational modes (often vendor proprietary), often results in a reference signal design associated with an unwarranted overhead cost.